The present invention relates to a semiconductive rubber roller used as a development roller, electrostatic charging roller and image-transfer roller in photocopying machines, printers, facsimile machines and the like.
Semiconductive rubber rollers having a semiconductive foamed rubber layer provided coaxially on and around a shaft of an electro-conductive material such as metals are known and widely employed, for example, as a development roller in photocopying and printing machines. FIG. 2 of the accompanying drawing is a schematic cross sectional illustration of such a development system in a photocopying machine in which the development roller 13 which is a semiconductive rubber roller bearing triboelectrically chargeable toner particles in a thin layer on the outer surface is provided to effect development and visualization of the latent image formed therein. The development system of FIG. 2 comprises, besides the development roller 13, a photosensitive drum 11, LEED array 12, toner-carrier roller 14, transfer roller 15, cleaning roller 16, charging roller 17 and development blade 18 as a toner layer limiter on the recording paper sheet 19.
The semiconductive rubber rollers used in the above mentioned applications are required to satisfy various requirements, for example, for adequate electric conductivity, weatherability to withstand adverse ambient conditions, relatively low hardness of the rubber layer, good triboelectric charging behavior and so on. These requirements are satisfied in conventional semiconductive rubber rollers by using a semiconductive rubber composition prepared from a non-conductive rubber compound such as urethane rubbers, NBRs, silicone rubbers and the like with a conductivity-imparting particulate material including electron-conductive agents such as carbon blacks, graphite powders and the like and ion-conductive fillers.
A serious problem encountered in the use of a conventional semiconductive rubber roller prepared by using a urethane rubber, NBR and the like as the base rubber compound of the semiconductive rubber is that, since the rubber composition is formulated with admixture of certain liquid additives such as process oils and softening agents, the rubber hardness of the rubber layer of the roller cannot be high and bleeding of the liquid additives is sometimes unavoidable. A conventional countermeasure taken for preventing bleeding of the liquid additives is to provide a protecting coating layer of a synthetic resin such as a urethane resin on the surface of the semiconductive rubber layer of the roller. While being effective for preventing bleeding, however, this method is not quite effective for improving the weather-ability or durability of the semiconductive rubber roller in an adverse ambient condition. When the semiconductive rubber roller is prolongedly exposed to an atmosphere of a high temperature and high humidity, for example, a reaction of hydrolysis proceeds in the resinous ingredient constituting the protective coating layer resulting in eventual sticking of the semiconductive rubber roller onto the latent image-carrying surface or in uncontrollable great fluctuation of the electric properties of the roller surface depending on the variations in the temperature and humidity. Since the electric chargeability of toner particles depends on the ambient conditions, furthermore, charge distribution in the toner is unavoidably broadened so that variations in the temperature and humidity sometimes result in occurrence of fogging in the background areas of printed images due to deficiency in charging on the surface of the semiconductive rubber roller.
On the other hand, development rollers of which the semiconductive rubber layer is formed from a silicone rubber-based semiconductive rubber composition have a problem that, though advantageous in respect of good stability against variations of the ambient conditions and absence of ambience-dependent fluctuations of the electric resistivity, the surface of the latent-image carrier and the toner particles are sometimes stained due to bleeding of the liquid ingredients contained therein adversely affecting development performance.
As a countermeasure for these problems in the prior art, the inventors previously proposed a method for preventing stain on the surface of the latent-image carrier and toner particles by decreasing the content of low molecular-weight fractions in the organopolysiloxane constituting the silicone rubber forming the semiconductive foamed rubber layer of the roller. This method was successful to some extent in providing semiconductive silicone rubber rollers of low stain.
The above mentioned proposal for decreasing the content of low molecular-weight fractions in the organopolysiloxane constituting the semiconductive silicone rubber is performed by conducting the polymerization procedure to prepare the organopolysiloxane gum from an oligomeric organopolysiloxane under such conditions as to decrease the residual amount of the unreacted low molecular-weight organopolysiloxane as far as possible or by removing the low molecular-weight fractions in the organopolysiloxane as polymerized by extraction with an organic solvent.
The former method mentioned above, however, has a problem that, when such a measure is undertaken, the number of the cross-linking points available for curing of the silicone rubber cannot be large enough so that the mechanical strength of the cured silicone rubber is correspondingly low resulting in a decrease in the durability of the thus prepared semiconductive silicone rubber roller.
The latter method mentioned above also has a problem that, as a consequence of the extraction with an organic solvent, the silicone rubber is unavoidably subject to degradation of various properties also resulting in a decrease in the durability of the semiconductive silicone rubber roller.